The mountain range located in Iran that serves as a barrier to the Caspian Sea is the Alborz Mountains. This range extends across northern Iran and is known for its stunning peaks, including Mount Damavand, which is the highest in the country. The Alborz Mountains play a critical role in Iran's climate, influencing rainfall patterns and providing a source of freshwater for the region. They are also a popular destination for hiking, skiing, and other outdoor activities.